Metallic alloy coatings are widely used to create functionality that is not possessed by the underlying material. A good example is the case of thermal barrier coating (TBC) systems which are used for the thermal and oxidation protection of the high temperature components used in advanced gas turbine and diesel engines to increase engine operating temperatures (and therefore improve engine efficiency) and to improve component durability and life. The TBC's currently in use are multi-layer systems consisting of a Zirconia based top layer that thermally protects the superalloy component, and an underlying bond coat which improves the top coat adhesion. The bond coat is typically an aluminum containing alloy such as MCrAlY (M=Ni and/or Co) or an aluminum based intermetallic such as a nickel or platinum aluminide. This layer is well bonded to a thin (˜1 μm) thermally grown (aluminum) oxide (TGO) layer which impedes oxidation and hot corrosion of the underlying component. This TGO layer is formed on the surface of the aluminum-rich alloy bond coat layer. The TBC systems currently in use are multilayer systems consisting of an yttria partially-stabilized zirconia (YSZ) top layer that thermally protects the superalloy component, and an underlying MCrAlY (M=Ni, Co) or nickel aluminide bond coat which improves the YSZ adhesion.
Bond coats have conventionally been applied using a variety of techniques depending on the materials system used. For example MCrAlY bond coats are applied using low pressure plasma spray (LPPS), electron beam physical vapor deposition (EB-PVD) and sputtering. The aluminide bond coats are typically applied using a diffusion based process. Such processes include pack cementation, vapor phase aluminiding (VPA), or chemical vapor deposition (CVD). The diffusion processes result in a bond coat with two distinct zones; an outer zone which contains an oxidation resistant phase (such as beta-NiAl) and a diffusion zone which consists entirely of the oxidation resistant phase and secondary phases (such as gamma prime, gamma, carbides and sigma).
The primary function of the bond coat is to form a thin, slow growing, alpha alumina oxide layer (TGO) which protects the underlying component for oxidation and corrosion. This function is dependent on the composition and morphology of the coating. The composition is critical to the formation of the TGO layer for two reasons. The first is the need to have an aluminum level high enough to support the continued growth of the protective aluminum oxide layer during the lifetime of the coating system. As a TGO grows in service the aluminum content is continually decreased. When the aluminum content falls below a critical level, nonprotective oxides begin to form which lead to spallation of the TGO layer. Thus, a large aluminum reservoir is desired. TGO formation can also be effected by minor alloy additions which may occur as a result of inter-diffusion between and bond coat and the superalloy substrate. Such elements can increase the growth rate of the TGO layer and may promote the formation of unwanted, nonprotective oxide scales. Ideally, inter-diffusion between the bond coat and the superalloy should be limited both during the formation of the bond coat and during service of the component.
The surface morphology of a bond coat can also effect TGO growth. For example, a dense coating free of open porosity is required to form a protective scale on the coating surface. Open porosity results in internal oxidation of the bond coat and oxidation of the underlying component. Another key morphological feature of the bond coat is a grain size. The presence of insoluble particles has been used to create fine grain sizes (x) which are thought to increase the lifetime of TBC systems. Higher static and creep strength bond coats are desired as they limit the thermomechanical phenomena which lead to failure of the TBC system.
The conventional processes currently in use for applying bond coats are limited by several drawbacks. None of the processes adequately control the composition of the bond coat. For the diffusion based processes, inter-diffusion between the bond coat and the substrate and the inability of these approaches to systematically add minor alloying additions to the bond coat are serious impediments. In EB-PVD, poor composition control results when materials with widely varying vapor pressures are deposited. A second drawback of the conventional processes is the high cost of such processes. Sputter deposition suffers from low vapor creation rates. EB-PVD has a very low process efficiency which results in low deposition rates and an uneconomical process. Furthermore, the line-of-sight deposition makes it difficult to coat non planar samples. Plasma spray coatings are often not fully dense and thus the oxidation resistance of such coatings is poor.
There is therefore a need in the art for a low cost deposition approach for applying bond coats which have the desired compositions which are difficult to deposit using conventional approaches. Further, there is a need in the art for a deposition approach for applying bond coats that exhibit condition control, such as morphology composition and grain size of deposited bond coats.
In all such cases, the ability to deposit compositionally controlled coatings efficiently, uniformly, at a high rate, with high part throughput, and in a cost-effective manner is desired. Some illustrative examples of deposition systems are provided in the following applications and patents and are co-assigned to the present assignee 1) U.S. Pat. No. 5,534,314, filed Aug. 31, 1994, entitled “Directed Vapor Deposition of Electron Beam Evaporant,” 2) U.S. Pat. No. 5,736,073, filed Jul. 8, 1996, entitled “Production of Nanometer Particles by Directed Vapor Deposition of Electron Beam Evaporant,” and 3) U.S. application Ser. No. 09/634,457, filed Aug. 7, 2000, entitled “Apparatus and Method for Intra-layer Modulation of the Material Deposition and Assist Beam and the Multilayer Structure Produced Therefrom”. These applications are hereby incorporated by reference herein in their entirety. The present invention discloses, among other things an apparatus and a method for applying a bond coating(s) on a substrate(s) in an improved and more efficient manner.
Other U.S. Patents that are hereby incorporated by reference herein in their entirety include the following:    U.S. Pat. No. 6,096,381, Zheng (2000)    U.S. Pat. No. 6,123,997, Schaeffer et al. (2000)    U.S. Pat. No. 6,153,313, Rigney et al. (2000)    U.S. Pat. No. 6,168,874, Gupta et al. (2001)    U.S. Pat. No. 6,255,001, Darolia (2001)    U.S. Pat. No. 6,258,467, Subramanian (2001)    U.S. Pat. No. 6,273,678, R. Darolia (2001)    U.S. Pat. No. 6,291,084, Darolia et al. (2001)    U.S. Pat. No. 6,306,524, Spitsberg et al. (2001)